Institute of Fundamental Technological Research

Within the framework of the EuroFusion WPMAG project, an automatic laser-welding line was constructed to produce a 1 km long, empty stainless steel jacket demonstrator for the React & Wind (RW) conductor for EU-DEMO. Four 500 m long C-profiles made of 316 L austenitic stainless steel were fabricated from ∼8 m long sections using the manual TIG (Tungsten Inert Gas) welding method. A series of experimental investigations was carried out on the welded samples, including ferrite content measurements, microhardness tests, residual stress measurements, and shape deviation assessments. The results revealed that part of the austenitic structure transformed into ferromagnetic phase—ferrite—around the heat affected zone (HAZ), with up to 7% ferrite observed in the laser welds and up to 10% in the TIG welds. Due to the relative magnetic permeability of ferrite (μᵣ > 1), electromagnetic (EM) forces will be present in that region of the jacket during magnet operation.
The microhardness measurements revealed an increased hardness in the welded region—up to 40.6%—due to material hardening and the presence of harder ferrite in the microstructure. Residual stresses were measured using the hole-drilling technique for both TIG and laser welds, revealing mostly compressive stresses in the TIG welds and tensile stresses in the laser welds. Considerable compressive stresses were introduced into the TIG welds during grinding. To assess the equivalent residual stress, a method of approximating the lower bound of the von Mises residual stress was proposed, revealing increasing values with the depth up to 1 mm, exceeding the initial yield stress at depths greater than 0.5 mm and reaching up to 425 MPa.
The shape deviations around the TIG weld reached 0.41 mm, with deformations toward the centre of the wide side of the jacket, resulting in a concave shape. Such deviations are considerable and could impact subsequent assembly steps of the superconducting Cable-In-Conduit Conductor (CICC).
This study presents a procedure for evaluating weld quality in conductor jackets, focusing on residual stresses, phase transformations, and welding-induced property changes.

Welded jacket, Residual stresses, RW conductor, EU-DEMO

Phase transformation driven by plastic strains is commonly observed in austenitic stainless steels. In the present paper, this phenomenon is addressed in connection with damage evolution. A three-dimensional constitutive model has been derived, and scalar variables for damage and the volume fraction of the transformed phase were used. The model was solved using Abaqus UMAT user defined procedure, as well as by means of simplified one-dimensional approach for a twisted circular bar. Large experimental campaign of tests was performed, including martensite content measurements within the cross-section and on the surface of the bar during monotonic and cyclic loading. Based on the residual angle of twist, damage variable was calculated. The global response of torque versus the angle of twist was measured as well. Comparison between the experimental results and the results obtained from the simplified one-dimensional approach and from the full three-dimensional approach are presented. It turns out that one-dimensional formulation agrees quite well with full three-dimensional model. Thus, much simpler approach can effectively be used. Moreover, experimental results agree well in terms of the martensite content evolution and relation: torque versus the angle of twist. Damage evolution is correctly predicted in terms of the maximum values. Lastly, the evolution of damage during cyclic torsion is discussed, as the experimental results indicate rather surprising effect of unloading modulus recovery after each reversion of twist direction

Cryogenic temperatures, plastic strain-induced phase transformation, torsion, damage evolution, constitutive model, martensite, austenitic stainless steels